1. Field of the Invention
The present invention relates to a two-cycle combustion engine for use mainly as a drive source for a small-size work machine such as, for example, a brush cutter.
2. Description of the Prior Art
It is conventional in a two-cycle combustion engine that, prior to scavenging of the combustion chamber with an air/fuel mixture, initial scavenging of the combustion chamber with air is carried out to suppress a blow-off of the air/fuel mixture through an exhaust port (see, for example, Japanese Laid-open Patent Publication No. 2000-136755). This known two-cycle combustion engine makes use of a carburetor of a type including an air flow passage, having an air control valve built therein, and an air/fuel mixture passage extending parallel to the air flow passage and having an air/fuel mixture control valve built therein. In this known construction, during an intake stroke of the engine, the air/fuel mixture can be introduced from the air/fuel mixture passage in the carburetor into a crank chamber through an air/fuel mixture passage in a intake tube and an air/fuel mixture supply passage in a cylinder block and, at the same time, an air can be introduced from the air flow passage in the carburetor into a scavenging passage in the cylinder block through an air passage in the intake tube, an air supply tube and a coupling tube. On the other hand, during a scavenging stroke the air is initially introduced into the scavenging passage, prior to introduction of the air/fuel mixture into the combustion chamber, to perform a leading scavenging to thereby suppress the blow-off of the air/fuel mixture through the exhaust passage.
It has, however, been found that since in the prior art two-cycle combustion engine, the carburetor includes therein the air flow passage and the air/fuel mixture passage having the air control valve and the air/fuel mixture control valve built therein, respectively, the carburetor tends to be complicated in structure and expensive. Also, two reed valves are required in the prior art two-cycle combustion engine in order to block an inflow of the combustion gases into the coupling tube, resulting in increase of the component parts used and, hence, increase of the cost of manufacture. Also, since in the prior art two-cycle combustion engine the leading scavenging is carried out by the utilization of air, the timing at which the air/fuel mixture is subsequently introduced into the combustion chamber tends to delay or too much air tends to be sucked, eventually resulting in lack of acceleration.